The present invention relates to holographic displays and in particular to holographic displays which use an electrically addressable spatial light modulator to generate modulated light for projection onto an optically addressable spatial light modulator.
It is well known that a three-dimensional image may be presented by forming an interference pattern or hologram on a planer surface. The three-dimensional image is visible when the hologram is appropriately illuminated. Recently, interest has grown in so-called computer generated holograms (CGHs) which offer the possibility of displaying high quality images, which need not be based upon real objects, with appropriate depth cues and without the need for viewing goggles. Interest is perhaps most intense in the medical and design fields where the need for realistic visualisation techniques is great.
Typically, a computer generated hologram involves the generation of a matrix of data values (each data value corresponding to a light transmission level) which simulates the hologram which might otherwise be formed on a real planer surface. The matrix is applied to an Electrically Addressable Spatial Light Modulator (EASLM) which may be, for example, a two-dimensional array of liquid crystal elements or of acousto-modulators. Coherent light is directed onto the EASLM using for example a laser such that the resulting output, either reflected from the EASLM or transmitted through the EASLM, is a modulated light pattern.
In order to produce a three-dimensional image of usable size and viewing angle, the EASLM typically has to have a large number of pixels, e.g. 1010. In addition, the pixels of the EASLM must be positioned relative to one another with a high degree of accuracy. The device must also be capable of modulating coherent light, e.g. produced by a laser. These requirements are extremely demanding and expensive to achieve in practice.
An alternative approach is presented in GB2330471A, referred to as Active Tiling(trademark), which describes the use of a relatively small EASLM in combination with a relatively large Optically Addressable Spatial Light Modulator (OASLM). The holographic matrix is subdivided into a set of contiguous tiles, with the data for each tile being passed in turn to the EASLM. The OASLM comprises a sheet of bistable liquid crystal (in one example the liquid crystal is a ferroelectric liquid crystal) which is switched from a first to a second state by incident light. Guide optics, disposed between the EASLM and the OASLM, cause the output of the EASLM to be stepped across the surface of the OASLM. The bistable nature of the OASLM liquid crystal means that the portion of the OASLM onto which an image tile is projected, remembers that image until such time as the OASLM is reset by the application of an electrical voltage. It will be appreciated that providing a reset voltage is applied only at the end of a complete scan, immediately prior to reset the OASLM will have xe2x80x9cstoredxe2x80x9d in it a replica of the complete holographic matrix.
The requirement of the display of GB2330471A for the OASLM to have memory, is quite demanding and is expensive to implement in practice. Furthermore, the need to reset the OASLM at the end of each scan, requiring as it does the use of surface electrodes, adds complexity and therefore yet more expense to the holographic display.
It is an object of the present invention to overcome or at least mitigate the above noted disadvantages. This and other objects are achieved at least in part by providing a holographic display for displaying a computer generated hologram comprising sub-holograms, the display having an OASLM where the modulation property of the OASLM is monostable.
According to a first aspect of the present invention there is provided a holographic display comprising:
a light source;
an Electrically Addressable Spatial Light Modulator (EASLM) in the path of the light source;
an Optically Addressable Spatial Light Modulator (OASLM) having a monostable light modulation property which is changed from a first state to a second state by incident light; and
light guiding means arranged to guide light from the EASLM to each of a set of tiled regions of the OASLM in turn.
The present invention takes advantage of the xe2x80x9cmemoryxe2x80x9d which is inherent in the human eye. Providing that the light output from each OASLM tile is modulated with sufficient amplitude, an observer will remember the image produced by that tile at least for the time it takes to complete a scan of the OASLM.
It will be appreciated that the holographic image data temporarily recorded at the OASLM may be read out in a number of different ways. For example, in a transmission mode, xe2x80x9creadoutxe2x80x9d light is directed onto the OASLM from the same side as the xe2x80x9cwritexe2x80x9d light and passes through the OASLM. Providing that the readout and write light are of different wavelengths, the write light can be prevented from reaching the viewer by means of optical filters. In a reflection mode, the readout light illuminates the OASLM from the opposite side of the OASLM than the write light. The write light is prevented from passing through the OASLM by an absorbing layer.
In one embodiment of the invention, the EASLM is arranged to be driven in turn by a sequence of sub-holograms which together (e.g. when tiled) form a complete holographic image frame. The light guiding means is synchronised with the tile update rate of the EASLM such that the sub-holograms are mapped onto respective tiles of the OASLM. Preferably, the sub-holograms are such that the OASLM produces a discrete set of wavefronts which converge on an object point.
The monostable nature of the modulation property of the OASLM is preferably such that the property decays from said second state to approximately said first state in a time which is of the same order as the frame update rate, i.e. the time taken to display a complete set of sub-holograms. More preferably, the modulation property decays to approximately the first state in a time which is less than the frame update rate. Due to the monostable nature of the modulation property, there is no requirement to re-set the modulation property at the end of a complete scan (i.e. a frame update). It will also be appreciated that the availability and affordability of monostable OASLMs is greater than that of bistable OASLMs.
It is noted that said tiled regions may be overlapping or contiguous with one another or may be spaced apart.
According to a second aspect of the present invention there is provided a method of displaying a hologram, the method comprising:
driving an Electrically Addressable Spatial Light Modulator (EASLM) with computer generated holographic data;
directing light onto the EASLM; and
guiding light output from the EASLM to an Optically Addressable Spatial Light Modulator (OASLM) having a monostable light modulation property, said light being guided to each of a set of tiled regions of the OASLM in turn.